A simplified universal differential or single ended phono preamp

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DIY is hobby that takes time (to enjoy). ;)

I decided to discard the heatsink and dump the heat to the case instead.
So on the "back" side, I added a board for PS caps and R-C for the output followers.
The side facing the preamp is of course a flooding Gnd plane.


Patrick

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Um, no.

RCA ground to chassis is a complete failure a lot of the time. It really depends on whether it's to the input or output, and what's between them. "pin 1" is a failure of an argument if you think every RCA ground should be direct to chassis. It just isn't that simple.

For what it's worth (coming from a relative novice)

I always use isolated RCA connectors, if I'm going to use them at all.

Ground currents are far more easily routed.
 
This is a universal differential or single ended output to compliment the phono front end. Bob Cordell used IC op-amps to provide gain and additional RIAA time constants here only gain is needed. It has been suggested that the impedances of the original front end could be scaled to get 40db of gain directly. Since this involves a lot of recomputing and checking I leave ths as a later experiment.
For the first circuit the options are:
1) Simply use one of the outputs with choice of phase. I recommend gain of 5 (14dB) to bring the gain to 40dB. Of course you could use just one 1512 for max economy.
2) As shown this is fully differential in and out so the gain is now 46dB total. This also has a 3db noise advantage, the 1512's are effectively in parallel for noise since adding uncorrelated noise does not care about phase.
3) Not shown but it is possible to add a simple servo from each in-amp output to the reference pin (5), a simple op-amp integrator will do.
The second circuit sums the two in-amps with no extra components for a dedicated single ended output, giving again 46dB of gain and 3dB less noise. All these options could easily be a few jumpers or dip switch. We published this circuit as a design idea in EDN over 30yr. ago and it actually won the $150 best of prize!


Hello,
I hope my question is not way off topic... I'm trying to build a high gain stage with the THAT INAs as shown in the above post. I've already built a quick prototype and I need to address the dc offset. It is very likely to end up with coupling capacitors and there is no problem with that but since I'm in the designing phase :eek: I thought to try a DC servo as well. I found a schematic supposed from an older datasheet - attached - but my knowledge only allows copy and paste so, I would like to ask if this is the right circuit to use. Also, it would be more practical to use a double opamp in DIP8. I would appreciate any recommendations.



Thanks in advance.
 

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I need to provide current limiting for emitter follower outputs of a differential phono stage connecting to a single ended line stage to prevent destruction of transistors (on signal cold) on start-up charging of output caps. I will use a simple parallel resistor/Zener circuit. Is there any benefit in connecting the upstream end of the Zener to the base rather than to the emitter?
 
This is one of the reasons why I stayed with the original circuit and only increased the gain to 60dB at 1kHz.
This was meant to cater for MC without the need for an additional headamp.
Apart from a quad of 2SK372GR for the input stage, one can do this with only 2SK170s or even 2SK117.
2SK117 is of course still available from Mouser in SMD as 2SK209GR.
(2x 2SK209 is close enough to 1x 2SK170, or 1/2x of 2SK372)

Although I used a pair of 2SJ74s for the cascode, one can equally use 2SA1312BL to replace.
At least in simulation there is little penalty.
So this can still be built without having to source 2SJ74s.
If one would reduce gain for MM only, then even 2SK372 is not necessary, and 2SK170 will do.


Patrick

would you please share the detail schmatic? really apprecaite it.
 
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